It has been known that, due to its structural characteristics, in a diarylvinylene compound, that is a compound wherein two aromatic rings are bonded via vinylene, such as a compound represented by formula (I) described below [hereinafter, referred to as Compound (I)], for example, the vinylene moiety (a carbon-carbon double bond; conjugated olefin) is isomerized (isomerization from a Z-isomer to an E-isomer, or from an E-isomer to a Z-isomer), or two molecules thereof are intermolecularly bonded at the vinylene moiety to form a diner (dimerization). It has been further known that, for example, a xanthine derivative having a styryl structure in its structure represented by formula (IA) described below [hereinafter, referred to as Compound (IA)], more specifically, (E)-8-(3,4-dimethoxystyryl)-1,3-diethyl-7-methyl-3,7-dihydro-1H-purine-2,6-dione (hereinafter, referred to as Compound 1) is unstable particularly under irradiation of light and is easily isomerized at the vinylene moiety of the styryl structure, and that it is particularly unstable to light in a state of a solution [Bioorg. Med. Chem. Lett., vol. 7, p. 2349-2352 (1997)]. On the other hand, it has been known that Compound (IA) or a pharmaceutically acceptable salt thereof shows adenosine A2 receptor antagonistic activity and is therefore useful for the treatment of various diseases induced by hyperactivity of adenosine A2 receptors such as Parkinson disease, senile dementia, depression or the like (e.g. EP0590919).
Additionally, for example, tablets containing lactose, potato starch, hydroxypropyl cellulose and Compound 1, and the like have been known (Japanese Published Unexamined Patent Application No. 211856/94).
As mentioned above, solid formulations containing Compound (I) or a pharmaceutically acceptable salt thereof, which have a general composition, have the problems of an increase in impurities caused by (a) isomerization, (b) dimerization or the like at the vinylene moiety due to the structural characteristics of the compound. Therefore, the greatest care is needed for its handling in the processes of preparing formulations, in preparing medicines in hospitals or pharmacies, in the storage of formulations, and the like. Solid formulations containing Compound (IA) or a pharmaceutically acceptable salt thereof, which have such general composition as described in Japanese Published Unexamined Patent Application No. 211856/94, have problems that (c) the hardness is insufficient, (d) the disintegration time is long, (e) the dissolution is likely to be delayed, and the like, in addition to the above problems.